When the winklevosses were prudent to make a wise investment in BTC in 2012, why did they not sell a large percentage in the great BTC rally of 2013?

It's widely speculated that they own 1% of coins in circulation. Which means during the rally their BTC holdings were worth at least $100 million. When their combined net worth is about $200 million why would they not sell out. It would have increased their net worth by 50% overnight.
Did the BTC market not have enough liquidity to make them whole, if they decided to cash out a percentage of their holdings?
There are people on the forums who were apparently able to cash out $1MM worth of BTC on finex.